Digital audio player chains known in the art usually comprise digital interpolation filters followed by Digital-to-Analog Converters (DAC) and programmable gain amplifiers (PGA) that drive acoustic transducers. For example, a MP3 player includes two audio chains (left and right) that drive the left and right headphone transducers, respectively. Multimedia cellular phones include at least one additional chain to drive an earpiece for phone conversation. In the case more analog processing is desired or there is a need to connect extra transducers (for example, high power loudspeakers) a mixer and/or a multiplexer block can be included into the chain before the PGA drivers.
As known, in portable devices, e.g. MP3 players, cellular phones, PDAs (Personal Digital Assistant), portable computers, it is of utmost importance to reduce the power consumption of each block of the audio chain in order to increase as much as possible the listening (playback) time before a battery charging is required. Several solutions have been implemented to decrease the current consumption of the DAC and the driver amplifiers under the mA range without impacting their performances. However, one of the aspects that is not usually addressed is the current dissipation due to offset voltages that are present in the chain.
In fact, the offset voltage at the output of an audio chain, i.e. at earpiece driver output, can be high, for example, 10 mV because it comes from both the DAC and the PGA driver. Moreover, such offset can be amplified if the PGA gain is greater than 1. Assuming that the load impedance of the earpiece is typically 8 Ohm, the current consumption is 10 mV/8 Ohm=1.25 mA, that is comparable or even higher than the whole current consumption of the driver amplifier plus the DAC itself. Thus, a way to reduce the offset voltage generated in the whole audio chain is mandatory for future portable devices.
There are a few existing solutions used to reduce the offset generated in the audio chain.
A first known solution is based on trimming the offset voltage at silicon wafer level. Such trimming can be performed by using laser, fuses, Few Times Programmable (FTP) or One Time Programmable (OTP) circuits. It consists in measuring the offset at the output of the driver amplifiers by a testing machine, and then storing an offset, equal and opposite to the one to be cancelled, somewhere in the chain path. Unfortunately, such methodology requires extra process steps and extra testing time, i.e. extra manufacturing costs. Moreover, this solution reduces the offset voltage close to zero, only for a fixed gain of PGA drivers (the gain used during the wafer trimming), while it becomes ineffective when the manufacturer of the mobile phones and MP3 players change the PGA gain.
A second known solution is based on auto-calibration of circuits at power-up of the devices that include the audio player chain. Particularly, the offset voltage is cancelled by an extra on-chip circuit that is able to measure and calibrate the offset voltage according to a predefined algorithm. This solution is effective for any PGA selected gain if a power-up sequence is allowed before the gain selection. However, if a user changes the PGA gain during audio listening, it is not possible to start a power up sequence, thus even this solution become ineffective.
A third known solution is a circuit structure 300 shown in FIG. 3. In more detail, a high-pass filter, formed by capacitors C and resistors R1, is inserted at the input of the PGA driver amplifier 3 to remove the offset voltage of the DAC and to avoid the amplification of the offset of the driver itself, thus obtaining offsets lower than 1 mV at the driver output. This circuit structure has been widely used for video or voice applications. However, it can only be implemented with discrete circuit components and so it cannot be used for audio applications (audio bandwidth is from 20 Hz to 20 kHz). In fact, because the input impedance of the driver amplifier, i.e. the input resistance R1, is usually in the range of a few tens of kΩ, in order to obtain a suitable cut-off frequency of about 10 Hz, the value of the capacitor C should be of some hundreds of nF. It is known that such high capacitance values are too big to be integrated.
A reference to the above circuit in FIG. 3 can be found in the book “Elettronica Analogica” by Franco Zappa, page 94, FIG. 3.17, particularly, in relation to a single ended solution.